Recent chromosome banding techniques have greatly improved the precision with which human cytogenetics can be carried out. The mechanisms by which these banding procedures work is not fully understood. It is clear that they are sensitive to genetically inactive (heterochromatic) and genetically active (euchromatic) portions of the genome, and that chromosomal proteins play an important role. The following aspects of this problem are being investigated: 1. Methods are being developed to isolate constitutive heterochromatin, and inactive and active euchromatin. 2. The histone and non-histone proteins of these fractions are compared by SDS-gel electrophoresis. 3. The kinetics of dye binding to these fractions are determined and the role of base composition and protein content examined. 4. The role of protein in enhancing and quenching quinacrine fluorescence is being studied. Numerous other aspects of chromosome structure are also under investigation.